1. Field of the Invention
This invention relates to spark generation and timing apparatus for internal combustion engines and particularly to a stacked capacitive discharge ignition apparatus for use with a conventional high energy ignition (HEI) of an internal combustion engine having an ignition transformer connected in a circuit for discharging an ignition storage capacitor into the transformer.
2. Background and Description of Related Art
Commercial engines for vehicles are typically provided with a modern version of a conventional Kettering ignition in which an electronic controller is connected in circuit with an ignition transformer for providing a current through a primary coil for generating an ignition voltage across a secondary coil of the ignition transformer to provide a high energy spark producing potential. The conventional High Energy Ignition (HEI) as presently employed in many vehicles is shown in FIG. 1A in which an electronic controller receives a magnetic reluctance timing pickup signal relating to the rotation of the engine crankshaft for switching a Darlington-pair current switch (Q1) which is used to initiate current through the primary coil of an ignition transformer for storing energy in the primary coil. Previously, ignitions have used mechanical switch contacts, known as points, as the current switch for completing the current path between the power source, through the primary coil to electrical ground. As shown, a current sensing resistor may be provided in the circuit path for allowing the electronic controller to sense and control the current flowing through the primary coil of the ignition transformer. Accordingly, in an HEI system, when the current switch is closed, current starts flowing through the primary coil which sets up a magnetic field for storing energy in the ignition coil as the magnetic field is built up.
When the current through the ignition coil in an HEI ignition is interrupted, the energy stored in the magnetic field of the primary is then released through the secondary winding of the ignition transformer for generating a high energy spark producing potential. The turns ratio of the primary and secondary coil of the ignition transfer is typically a step up on the order of 100:1, which provides the high output voltage required at the spark gap of the spark plug connected through a distributor to the secondary winding coil of the ignition transformer. Thus, as the current switch opens, the magnetic field collapses and drives a high voltage current directed through the use of a high voltage distributor to the spark plugs of the internal combustion engine. The buildup and collapse of the magnetic field in the ignition transformer, however, is somewhat sluggish, providing the high energy over a relatively long ignition period, e.g., 1 to 3 milliseconds, which are not suitable for high engine revolutions per minute (rpms). Thus, the HEI system is not particularly effective at high rpms, whereas the CD system is relatively better suited for high rpm operation because of its high current and power in a short duration operation.
An alternate Capacitive Discharge (CD) ignition is known for providing very high power over relatively short duration ignition discharge periods, e.g., approximately 300 microseconds, through the use of discharging a high voltage capacitance through the ignition transformer. The CD ignition employs an ignition storage capacitor which is charged to several hundred volts, upon which a discharge switch is used under control of a timing controller for discharging the storage capacitor into the ignition transformer. With the capacitance and the leakage inductance of the ignition transformer forming a resonant circuit, the current delivered to the ignition transformer rises in a quarter of the resonance period to a maximum value of approximately 10 to 40 amperes, i.e., 100 to 400 milliamps at the secondary coil with a 100:1 turns ratio. Once the storage capacitor is fully discharged, building energy in the leakage inductance of the ignition transformer, the current in the leakage inductance represents a transfer of energy from the storage capacitor which is then stored in the magnetic field of the leakage inductance which is transferred to the secondary coil of the ignition transformer to provide the spark producing potential at the output of the transformer.
In the CD ignition, the primary inductance of the primary coil of the ignition transformer, however, represents a parasitic stored energy in the primary inductance which takes a relatively long time to decay. This current decays very slowly because of the low impedance and high inductance involved. Where the CD ignition is used at a high repetition rate (high rpms), the potential would exist for a direct current (DC) buildup in the ignition transformer which subtracts from the current provided at the output of the ignition transformer. Accordingly, an alternative path is provided with a diode, allowing the energy built up in the parasitic primary inductance to be reset quickly, however, the parasitic stored energy represents wasted energy nonetheless.
In the HEI ignition, on the other hand, the beneficial portion of the stored energy is provided from the primary inductance of the ignition transformer. With the HEI ignition, energy stored in the leakage inductance provides little benefit, and thus as illustrated in FIG. 1B is dissipated across the current switch of the HEI ignition already discussed. In the CD ignition, however, the beneficial portion of the stored energy is stored in the leakage inductance of the ignition coil. Thus as illustrated in FIG. 1C, the energy stored in the primary inductance acts as the parasitic inductance which provides little benefit, and is dissipated using a path provided across the primary inductance of the ignition transformer. It would be desirable therefore to provide an efficient ignition system taking advantage of the benefits of both the HEI and CD ignitions, while avoiding the respective circuit factors leading to parasitic components which generate potential stored energy losses.